Retort container, and hot melt adhesive therefor

ABSTRACT

A retort container comprises a container body structured and arranged for holding a food product, the container body defining an opening and a sealing surface surrounding the opening, and a lid bonded to the sealing surface of the container body to close the opening, the lid being bonded to the sealing surface with a hot melt adhesive. The adhesive comprises a polyamide resin blended with a second resin that raises the glass transition temperature of the adhesive above the glass transition temperature of the polyamide resin alone, the adhesive bonding the lid to the sealing surface with a temperature-dependent bond strength that is relatively low at room temperature and substantially higher at retort temperatures of 100° C. to 120° C.

BACKGROUND OF THE INVENTION

The invention relates to containers for products, and more particularlyrelates to retort containers for food.

Retort containers are containers that are hermetically sealed afterfilling with a food product, and are then heated to a temperature ofabout 100° C. to 120° C. for a period of time to ensure that allmicroorganisms in the container have been killed. The chief advantagesof retort containers is that they do not require refrigeration prior toopening, and they can be stored for long periods of time in theirinitially sealed condition. For many years, metal cans were thepredominant type of retort container.

More recently, plastic retort containers have been developed. Plasticcontainers are attractive for various reasons, includingmicrowavability. One of the challenges with plastic retort containers isthe closure system. More particularly, the container lid must behermetically sealed to the container with sufficient strength towithstand the elevated temperature and pressure conditions during retortsterilization; on the other hand, the seal strength must not be so highthat the consumer finds it difficult or impossible to remove the lid.Retort integrity and ease of opening are usually at odds with eachother, such that anything that is done to improve retort integrity has adeleterious effect on ease of opening, and vice versa.

Prior to the present invention, plastic retort containers have generallyincluded some type of membrane lid that is heat-sealed to the container.The difficulties of accurately controlling the seal strength of heatseals are well known. For instance, some heat seal materials such asSURLYN® (ionomer resin) have a relatively narrow heat sealingtemperature window within which the resulting seal strength is in adesirable range. If the sealing temperature is too low, the sealstrength may be inadequate to prevent failure during retort; if thetemperature is too high, the seal strength may be so great that theconsumer cannot remove the lid. It can be difficult to control thesealing temperature with sufficient accuracy to stay within the desiredwindow.

Other requirements for all-plastic retort containers and lids includehigh-barrier performance against water vapor and oxygen, and fastsealing speed. The high-barrier performance must be achieved without theuse of metal foil, which has commonly been used in many retortcontainers. Fast sealing speed is difficult to achieve with all-plasticlids because induction sealing cannot be used, and direct thermal heatsealing is slow because of the poor thermal conductivity of plastic.

BRIEF SUMMARY OF THE INVENTION

The invention addresses the above needs and achieves other advantages,by providing an all-plastic retort container wherein the lid isadhesively sealed to the container as opposed to being heat-sealed tothe container. The adhesive employed for sealing the lid to thecontainer is specially formulated to provide a temperature-dependentbond, such that at room temperature the bond strength is relatively low,while at retort temperatures of 100° C. to 120° C. the bond strength ishigher.

In one embodiment of the invention, a retort container comprises acontainer body structured and arranged for holding a food product, thecontainer body defining an opening and a sealing surface surrounding theopening, and a lid bonded to the sealing surface of the container bodyto close the opening, the lid being bonded to the sealing surface with ahot melt adhesive. The adhesive comprises a polyamide resin blended witha second resin that raises the glass transition temperature of theadhesive above the glass transition temperature of the polyamide resinalone, the adhesive bonding the lid to the sealing surface with atemperature-dependent bond strength that is relatively low at roomtemperature and substantially higher at retort temperatures of 100° C.to 120° C.

The hot melt adhesive preferably has a glass transition temperature(T_(g)) close to room temperature, for example about 20° C. to 40° C. Apreferred adhesive comprises a substantially amorphous polyamide resinas a primary component. The polyamide resin can have a T_(g) that isless than or equal to 0° C. An example of a suitable polyamide resin isUNI-REZ® 2617 available from Arizona Chemical of Jacksonville, Fla. Asecond resin is added to the polyamide resin in sufficient amount toraise the T_(g) of the resulting composition to about room temperature.The second resin can comprise a maleic modified rosin ester. An exampleof a suitable second resin is a maleic modified glycerine ester of talloil rosin such as SYLVACOTE® 7071 available from Arizona Chemical ofJacksonville, Fla.

As an example, the adhesive can comprise a polyamide resin and a maleicmodified rosin ester, wherein the maleic modified rosin ester comprisesabout 10 to about 20 weight percent of the adhesive.

The invention is applicable to a wide range of container types andconfigurations. As one example, the container body can be thermoformedfrom an all-plastic high-barrier sheet. The sheet can have a structure,for example, of {polypropylene/tie/EVOH/tie/polypropylene}. Thepolypropylene layers are good barriers against moisture, while theethylene vinyl alcohol copolymer (EVOH) layer provides oxygen barrierperformance. The sheet can be formed by coextrusion or other processes.

Alternatively, the container body can be formed by a coextrusionblow-molding process. For example, a parison can be coextruded as amulti-layer structure such as {polypropylene/tie/EVOH/tie/polypropylene}and then enclosed in a mold and inflated to form the container body.

The lid similarly can comprise a multi-layer structure such as{polypropylene/tie/EVOH/tie/polypropylene}. Other high-barriermulti-layer structures can be used for the container body and/or lid.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a schematic cross-sectional view of a thermoformed containerin accordance with one embodiment of the invention;

FIG. 2 is a schematic fragmentary cross-sectional view of the containerof FIG. 1 illustrating on an enlarged scale the lid sealed to thecontainer body to show the multi-layer structures of the lid andcontainer body, and the hot melt adhesive therebetween;

FIG. 3 is a schematic cross-sectional view of a blow-molded container inaccordance with another embodiment of the invention;

FIG. 4 is a schematic fragmentary cross-sectional view of a container inaccordance with yet another embodiment of the invention having a metalring and membrane lid closure; and

FIG. 5 is a schematic fragmentary cross-sectional view of the containerof FIG. 4 illustrating on an enlarged scale the lid sealed to the metalring to show the multi-layer structures of the lid and the hot meltadhesive for sealing the lid to the ring.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

FIG. 1 illustrates a first embodiment of the invention as applied to aplastic thermoformed container 10. The container 10 comprises acontainer body 12 and a lid 14. The container body 12 is formed bythermoforming a multi-layer high-barrier sheet. The container body 12has a bottom wall 16, and an upstanding side wall 18 that preferablyflares (i.e., gets larger in diameter) in the upward direction for easeof removal of the container body from the mold (not shown). At the upperend of the side wall 18 a radially extending flange 20 is formed as anextension of the side wall. The flange provides a sealing surface towhich the lid 14 is sealed.

With reference to FIG. 2, the multi-layer high-barrier sheet that formsthe container body can comprise, for example, a first layer 22 formingthe inner surface of the container and hence the upper surface of theflange 20, a second layer 24, and a third layer 26, wherein the secondlayer 24 is disposed between the first layer 22 and the third layer 26.Tie layers 23, 25 can be included between the first and second layersand between the second and third layers, respectively, for binding thelayers together. The first layer 22 can comprise a polypropylene, whichis a good barrier against moisture but is not particularly effectiveagainst oxygen. The second layer 24 can be an ethylene vinyl alcoholcopolymer (EVOH), which has good oxygen barrier properties but isdeleteriously affected by exposure to moisture. The third layer 26 cancomprise a polypropylene. Thus, the polypropylene layers providemoisture barrier performance and protect the EVOH layer from exposure tomoisture, and the EVOH layer provides oxygen barrier performance.Various other structures can be used for the multi-layer sheet fromwhich the container body 12 is formed, the invention not being limitedto any particular structure.

FIG. 2 also shows that the lid 14 can comprise a multi-layer structuresimilar to that of the container body. Thus, the illustrated lid has afirst layer 28, a second layer 30 and a third layer 32, which can bejoined together with tie layers 29, 31 if necessary or desirable. Thefirst and third layers can comprise a polypropylene, and the secondlayer can comprise EVOH. Alternative lid constructions can be used, theinvention not being limited to any particular lid structure.

The lid 14 is bonded to the flange 20 of the container body by a hotmelt adhesive 40. The adhesive 40 is formulated to provide atemperature-dependent bond strength between the lid 14 and the containerbody 12. More specifically, the bond strength is relatively low at roomtemperature so that the lid can be readily peeled from the containerbody, while the bond strength is higher at retort temperatures of about100° C. to 120° C. The adhesive preferably comprises a substantiallyamorphous polyamide-based composition having a glass transitiontemperature, T_(g), that is approximately room temperature (e.g., 20° C.to 40° C.). When amorphous polymers are near their glass transitiontemperature, they tend to lose their adhesiveness. Accordingly, byengineering the adhesive 40 so that its T_(g) is near room temperature,the bond strength provided by the adhesive is relatively low at roomtemperature. However, at retort temperatures, which are substantiallyabove T_(g), the adhesive has greater adhesiveness and hence provides astronger bond. In this manner, the lid can be bonded to the containerbody firmly enough to withstand retort without failure, and yet the lidcan easily be peeled off when the consumer desires to open thecontainer.

A suitable adhesive 40 can comprise a polyamide resin having a T_(g)that is less than or equal to 0° C., and a second resin that raises theT_(g) of the adhesive to about room temperature. The polyamide resin canbe, for example, the resin available from Arizona Chemical ofJacksonville, Fla., having the designation UNI-REZ® 2617. This resin hasa tensile modulus of 205 MPa (30,000 psi), a softening point (ASTM E 28ring and ball method) of 163° C. to 175° C., and a shear adhesionfailure temperature or SAFT (ASTM D 4498 method with a 1 kg. weight) of150° C.

The second resin can be a maleic modified rosin ester. An example of asuitable material is SYLVACOTE® 7071 available from Arizona Chemical.This material is a maleic modified glycerine ester of tall oil rosin,and its recommended applications include as a film-forming resin fornitrocellulose-based packaging-gravure inks that contain acetone, and inlacquer wood furniture sealer and sander coats. It has a softening point(ASTM E 28-67 ring and ball method) of 125° C.

The T_(g)-raising resin is added to the polyamide resin in sufficientquantity to raise the T_(g) of the resulting composition to about roomtemperature. The composition can also contain other components asdesired. As an example, the adhesive composition can comprise about 70to 95 weight percent, more preferably about 80 to 90 weight percent, ofpolyamide resin, and about 30 to 5 weight percent, more preferably about20 to 10 weight percent, of maleic modified rosin ester.

The adhesive should have a shear adhesion failure temperature (SAFT)above the expected temperature of retort, which is typically betweenabout 100° C. and about 120° C. While the addition of the maleicmodified rosin ester to the polyamide resin, which on its own has anSAFT of about 150° C., is likely to bring the SAFT of the compositiondown somewhat below 150° C., it should still be above 120° C.

As noted, the invention is not limited to any particular container typeor configuration. Thus, FIG. 3 shows an alternative container 110 havinga container body 112 that is formed by coextrusion blow molding. Thecontainer body includes a flange 120 to which the lid 114 is bonded by ahot melt adhesive as previously described. The container body is formedby coextruding a parison as a multi-layer structure, enclosing theparison in a mold, inserting a blow pin into the parison, and inflatingthe parison to fill the mold. As known in the art, blow molding allowsthe manufacture of container shapes that cannot be made bythermoforming, which is generally limited to tapered shapes that can belifted out of the open top end of the mold. In contrast, the molds usedin blow molding are formed in left and right halves such that the moldis opened by moving the mold halves apart, and hence molded articles ofnon-tapered and other shapes can be made, such as illustrated in FIG. 3.The container body wall can have a multi-layer structure similar to thatillustrated in FIG. 2, or can have alternative structures.

FIGS. 4 and 5 illustrate yet another type of container 210 to which theinvention is applicable. The container includes a container body 212,which can be of various materials and constructions including thosedescribed above or others, and a metal ring 250 that is affixed to theupper end of the container body. The ring is affixed to the containerbody by double-seaming, wherein a curled outer edge portion 252 of thering and a curled top edge portion 213 of the container body are rolledtogether to provide a hermetic joint therebetween. The ring includes anannular portion 254 that extends radially inwardly from the containerbody wall and terminates at a radially inner edge 256, which can becurled as shown. The upper surface of the annular portion 254constitutes a sealing surface to which the lid 214 of the container issealed by a hot melt adhesive 240.

The lid 214 can have various constructions. The exemplary constructionshown in FIG. 5 includes a first layer 228, a tie layer 229, a secondlayer 230, a tie layer 231, and a third layer 232. The first and thirdlayers can comprise polypropylene, and the second layer can compriseEVOH. In the various embodiments described above, materials other thanthose listed above can be used. For instance, instead of the maleicmodified glycerine ester of tall oil rosin, the adhesive can employother T_(g)-raising materials. Furthermore, while container body and lidstructures have been described that include a barrier structure of thetype {polypropylene/tie/EVOH/tie/polypropylene}, other structures can beused. For instance, the barrier structure can include aluminumoxide-coated polymer film, SiOx-coated polymer film, metallized film,polyacrylonitrile, and others.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A retort container, comprising: a container body structured andarranged for holding a food product, the container body defining anopening and a sealing surface surrounding the opening; and a lid bondedto the sealing surface of the container body to close the opening, thelid being bonded to the sealing surface with a hot melt adhesive, theadhesive comprising a polyamide resin blended with a second resin thatraises the glass transition temperature of the adhesive above the glasstransition temperature of the polyamide resin alone, the adhesivebonding the lid to the sealing surface with a temperature-dependent bondstrength that is relatively low at room temperature and substantiallyhigher at retort temperatures of 100° C. to 120° C.
 2. The retortcontainer of claim 1, wherein the second resin comprises a maleicmodified rosin ester.
 3. The retort container of claim 2, wherein theadhesive comprises about 10 to 20 weight percent of the maleic modifiedrosin ester.
 4. The retort container of claim 1, wherein the containerbody is formed entirely of polymer material.
 5. The retort container ofclaim 1, wherein the lid is formed entirely of polymer material.
 6. Theretort container of claim 1, wherein the lid and the container body eachis formed entirely of polymer material.
 7. The retort container of claim1, wherein the container body comprises a multi-layer body wall.
 8. Theretort container of claim 7, wherein the container body comprises athermoformed multi-layer sheet.
 9. The retort container of claim 7,wherein the container body comprises a coextrusion blow-moldedstructure.
 10. The retort container of claim 7, wherein the multi-layersheet includes a first polymer layer providing moisture barrierperformance and a second polymer layer providing oxygen barrierperformance.
 11. The retort container of claim 10, wherein the firstpolymer layer comprises polypropylene and the second polymer layercomprises ethylene vinyl alcohol copolymer.
 12. The retort container ofclaim 11, wherein the multi-layer sheet further comprises a thirdpolymer layer of polypropylene, the ethylene vinyl alcohol copolymerlayer being disposed between the polypropylene layers.
 13. The retortcontainer of claim 1, wherein the lid comprises a multi-layer structure.14. The retort container of claim 13, wherein the lid includes at leasta first polymer layer providing moisture barrier performance and asecond polymer layer providing oxygen barrier performance.
 15. A retortcontainer, comprising: a container body structured and arranged forholding a food product, the container body defining an opening and asealing surface surrounding the opening; and a lid bonded to the sealingsurface of the container body to close the opening, the lid being bondedto the sealing surface with a hot melt adhesive, the adhesive having aglass transition temperature of about 20° C. to 40° C., the adhesivebonding the lid to the sealing surface with a temperature-dependent bondstrength that is relatively low at room temperature and substantiallyhigher at retort temperatures of 100° C. to 120° C.
 16. The retortcontainer of claim 15, wherein the hot melt adhesive predominantlycomprises a polyamide resin.
 17. The retort container of claim 15,wherein the hot melt adhesive comprises a first resin having a glasstransition temperature substantially lower than 20° C., and a secondresin added to the first resin to raise the glass transition temperatureof the adhesive to about 20° C. to 40° C.
 18. The retort container ofclaim 17, wherein the first resin comprises polyamide.
 19. The retortcontainer of claim 17, wherein the second resin comprises maleicmodified rosin ester.
 20. The retort container of claim 17, wherein thefirst resin comprises polyamide and the second resin comprises maleicmodified rosin ester.
 21. A hot melt adhesive suitable for retortcontainers, comprising: a polyamide resin having a glass transitiontemperature less than or equal to about 0° C.; and a maleic modifiedrosin ester blended with the polyamide resin to form the hot meltadhesive, the maleic modified rosin ester being present in the hot meltadhesive in sufficient proportion to give the hot melt adhesive a glasstransition temperature of about room temperature.
 22. The hot meltadhesive of claim 21, wherein the polyamide resin comprises about 70 to95 weight percent of the adhesive and the maleic modified rosin estercomprises about 30 to 5 weight percent of the adhesive.
 23. The hot meltadhesive of claim 21, wherein the polyamide resin comprises about 80 to90 weight percent of the adhesive and the maleic modified rosin estercomprises about 20 to 10 weight percent of the adhesive.